The present invention relates an article, such as a micro-optical device, an information recording medium substrate, or the like, having a predetermined surface shape on a front surface of a substrate, and a mold (or die) used for producing the article by transfer under pressure.
The forming technique carried out by pressing a mold is widely used for mass-production of optical components such as CD-ROMs and other information recording media, planar micro-lenses (lens arrays in each of which a large number of micro-lenses are arrayed one- or two-dimensionally on a substrate), Fresnel lenses, diffraction grating devices, optical wave guide devices, and so on. A resin is generally used as the molding material. A glass material produced by a so-called sol-gel method is, however, used for application which requires severe limitation on heat resistance, weather resistance, and so on. In this sol-gel method, a mold (or a die) having a predetermined surface shape is pressed against a sol-gel material, which has been applied onto a predetermined substrate to have a predetermined thickness, under application of heat for a specific time. Then, the mold is released from the sol-gel material and then the sol-gel material is sintered or solidified again, so that an article having a desired surface shape and a predetermined thickness can be obtained.
In the sol-gel method as described above, however, pressing and heating need to be performed simultaneously in order to transfer the shape of the mold to the article with high accuracy.
On the other hand, silicon is an example of the material for the mold. As a method of producing a shape to be transferred, a surface of a single crystalline silicon wafer is coated with a photo resist and patterned by a photolithographic technique, so that V-shaped grooves are produced by wet-etching. Since anisotropy of etching speed with respect to crystal direction is utilized, silicon has an advantage in that shape control can be made with a high degree of reproducibility.
The thickness of the silicon wafer generally used is, however, thin in a range of from about 0.5 mm to about 1 mm. For this reason, when the mold is pressed against the article, there may occur a case in which the load is imposed unevenly on the mold at a point of time before the article is cured. In such a situation, the thin silicon wafer may be cracked and damaged. It may be possible to use a silicon wafer having a larger thickness instead. However, even if the thickness of the wafer is increased, the strength against cracking cannot be improved satisfactorily because of brittleness. Material cost increases. Therefore, a mold in which a substrate having a specific strength, such as a glass plate or the like, is stuck by an adhesive agent onto the rear surface of the thin silicon wafer having a molding shape has been used. In such a manner, a mold having both a highly accurate molding shape and strength against pressing has been put into practical use. Also, a technology, in which a silicon single crystal is processed to have a diffraction grating shape on a surface thereof to be used as a diffraction grating, is known (see Japanese patent Kokai Hei. 10-26707).
In the case where a sol-gel material is used as an article, the mold has to be heated as described above. Accordingly, when an adhesive agent, an adhesive tape, or the like, is used for bonding the silicon wafer and the glass substrate to each other, the adhesive agent, the adhesive tape, or the like, needs to have heat resistance to the temperature at the time of pressing. The temperature for heating may be not lower than 150° C. Most of general adhesive agents or adhesive tapes cannot be used at the aforementioned high temperature for production of the article. In addition, even if the adhesive agents or tapes could be used, adhesive power would be lowered with the passage of time. Accordingly, there has been a problem in reliability.
The specific examples experienced by the present inventors will be described below.
A mold was produced as follows. A 1 mm-thick silicon plate (25×25 mm) having a diffraction grating shape formed in its surface was bonded to a 7 mm-thick sheet of glass (borosilicate glass of #7740 made by Corning) having the same shape as the silicon plate, by a heat-resistant adhesive agent SE5080 made by Sin-Etsu Silicon Co., Ltd. This mold was used to press a sol-gel solution at 200° C. for 30 minutes (detailed conditions will be described later). After that, the sol-gel solution was intended to be released from the mold but the silicon plate and the sheet of glass were stripped one from the other, and the strength of the heat-resistant adhesive agent could not be maintained at the temperature of 200° C. Then, in order to perform mold release, a jig was inserted between the silicon plate and the article. This operation was however difficult and both the silicon plate and the article were damaged.
In another case, the same silicon plate and the same sheet of glass #7740 made by Corning as described above were bonded to each other by a double-coated adhesive tape #4390 (thickness: 0.13 mm) made by 3M Company. In the same manner as described above, the mold was used to press the sol-gel solution, and then the sol-gel solution was released from the mold. The PV (Peak to Valley) value of the diffraction grating shape formed in the front surface of the silicon plate was 0.17 μm when it was measured before bonding, whereas the PV value of the article was 0.45 μm, which indicated that the silicon plate was deformed because of the deformation of the double-coated adhesive tape at the time of molding.
In fact, when a diffraction grating manufactured from a silicon single crystalline is mounted to an apparatus to be used, a silicone plate processed to have a finely surface-relief shape must be thin in view of the cost. In this case, however, it is difficult to fix the diffraction grating without warp and deformation of the substrate. The use of a thick silicon plate results in the cost increase as mentioned above.